Chapter 11 of Database Design, Application Development and

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Transcript Chapter 11 of Database Design, Application Development and

Overview - Stored Procedures and
Triggers
Outline
Database programming language
background
 Stored procedures
 Triggers

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Motivation for Database
Programming Languages
Definition: a procedural language with an
interface to one or more DBMSs. The
interface allows a program to combine
procedural statements with nonprocedural
database access.
 Efficiency and portability

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Design Issues
Language style: call-level vs. statementlevel interface
 Binding: static vs. dynamic
 Database connection: implicit vs. explicit
 Result processing: data types and
processing orientation

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Language Style
Call-level interface: a set of procedures
and a set of type definitions for
manipulating the results of SQL statements
 Statement-level interface: changes to the
syntax of a host programming language to
accommodate embedded SQL statements
 Most DBMSs support both interfaces
 ODBC and JDBC are widely used calllevel interfaces

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Binding


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Association of access plan with an SQL
statement
Static binding: association at compile time
Dynamic binding: association at run time
Binding options:
– Static and dynamic for statement-level interface
– Dynamic for call-level interface
– Reuse of access plans for repetitively executed
statements in a program
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Database Connection
Implicit for stored procedures and triggers
because they are part of a database
 External programs: explicit connection

– CONNECT statement or procedure
– Web address or database identifier
– Database identifier is more flexible
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Results Processing
Data type mapping
 Processing orientation

–
–
–
–
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SELECT USING for single row results
Cursor for multiple row results
Cursor is similar to a dynamic array
Interface provides statements or procedures to
declare, open, close, iterate (position), and
retrieve values
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Overview of PL/SQL
Proprietary database programming
language for Oracle
 Widely used language
 Java style syntax with a statement level
interface
 Use PL/SQL for writing stored procedures
and triggers

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User Identifiers in PL/SQL
Provide names for variables and constants
 Not case sensitive
 Restrictions

–
–
–
–
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At most 30 characters
Must begin with a letter
Must be unique
Allowable characters are letters, numbers, _,
#, and $
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PL/SQL Constants
Numeric constants: whole numbers, fixed
decimal numbers, and scientific notation
 String constants: use single quotes; case
sensitive
 Boolean constants: TRUE, FALSE
 NULL: constant for every data type
 No string constants: use the To_Date
function to create string constants

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PL/SQL Data Types
String: CHAR(L), VARCHAR2(L)
 Numeric: INTEGER, DECIMAL(W,D),
FLOAT(P). SMALLINT
 Logical: BOOLEAN
 DATE: stores both date and time

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Variable Declaration Examples
DECLARE
aFixedLengthString
CHAR(6) DEFAULT 'ABCDEF';
aVariableLengthString
VARCHAR2(30);
anIntegerVariable
INTEGER := 0;
aFixedPrecisionVariable DECIMAL(10,2);
-- Uses the SysDate function for the default value
aDateVariable
DATE DEFAULT SysDate;
-- Anchored declarations
anOffTerm Offering.OffTerm%TYPE;
anOffYear Offering.OffYear%TYPE;
aCrsUnits Course.CrsUnits%TYPE;
aSalary1 DECIMAL(10,2);
aSalary2 aSalary1%TYPE;
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Assignment Examples
aFixedLengthString := 'XYZABC';
-- || is the string concatenation function
aVariableLengthString := aFixedLengthString || 'ABCDEF';
anIntegerVariable := anAge + 1;
aFixedPrecisionVariable := aSalary * 0.10;
-- To_Date is the date conversion function
aDateVariable := To_Date('30-Jun-2003');
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IF Statement Format
IF-THEN Statement:
IF condition THEN
sequence of statements
END IF;
IF-THEN-ELSE Statement:
IF condition THEN
sequence of statements 1
ELSE
sequence of statements 2
END IF;
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CASE Statement Format
CASE Statement (Oracle 9i only):
CASE selector
WHEN expression1 THEN sequence of statements 1
WHEN expression2 THEN sequence of statements 2
WHEN expressionN THEN sequence of statements N
[ ELSE sequence of statements N+1 ]
END CASE;
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Formats of Iteration Statements
FOR LOOP Statement:
FOR variable IN BeginExpr .. EndExpr LOOP
sequence of statements
END LOOP;
WHILE LOOP Statement:
WHILE condition LOOP
sequence of statements
END LOOP;
LOOP Statement:
LOOP
sequence of statements containing an EXIT statement
END LOOP;
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Common SQL *Plus Commands
CONNECT: login to a database
 DESCRIBE: list table details
 EXECUTE: execute statements
 HELP: lists column details
 SET: assigns values to SQL *Plus
environment variables
 SHOW: displays error details
 SPOOL: send output to a file

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PL/SQL Blocks
• Anonymous blocks to test procedures and triggers
• Named blocks for stored procedures
Block Structure:
[ DECLARE
sequence of declarations ]
BEGIN
sequence of statements
[ EXCEPTION
sequence of statements to respond to exceptions ]
END;
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Anonymous Block Example
SET SERVEROUTPUT ON; -- SQL Plus command
-- Anonymous block
DECLARE
TmpSum
INTEGER;
TmpProd
INTEGER;
Idx
INTEGER;
BEGIN
TmpSum := 0;
TmpProd := 1;
-- Use a loop to compute the sum and product
FOR Idx IN 1 .. 10 LOOP
TmpSum := TmpSum + Idx;
TmpProd := TmpProd * Idx;
END LOOP;
Dbms_Output.Put_Line('Sum is ' || To_Char(TmpSum));
Dbms_Output.Put_Line('Product is ' || To_Char(TmpProd));
END;
/
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Motivation for Stored Procedures
Compilation of programming language
statements and SQL statements
 Management of dependencies by the
DBMS
 Centralized management of procedures
 Development of more complex functions
and procedures
 Usage of DBMS security system for stored
procedures

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Format of PL/SQL Procedures
CREATE [OR REPLACE] PROCEDURE ProcedureName
[ (Parameter1, …, ParameterN) ]
IS
[ sequence of declarations ]
BEGIN
sequence of statements
[ EXCEPTION
sequence of statements to respond to exceptions ]
END;
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Simple Procedure Example
CREATE OR REPLACE PROCEDURE pr_InsertRegistration
(aRegNo IN Registration.RegNo%TYPE,
aStdSSN IN Registration.StdSSN%TYPE,
aRegStatus IN Registration.RegStatus%TYPE,
aRegDate IN Registration.RegDate%TYPE,
aRegTerm IN Registration.RegTerm%TYPE,
aRegYear IN Registration.RegYear%TYPE) IS
-- Create a new registration
BEGIN
INSERT INTO Registration
(RegNo, StdSSN, RegStatus, RegDate, RegTerm, RegYear)
VALUES
(aRegNo, aStdSSN, aRegStatus, aRegDate, aRegTerm,
aRegYear);
dbms_output.put_line('Added a row to the table');
END;
/
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Exception Example
CREATE OR REPLACE PROCEDURE pr_InsertRegistration
(aRegNo IN Registration.RegNo%TYPE,
aStdSSN IN Registration.StdSSN%TYPE,
aRegStatus IN Registration.RegStatus%TYPE,
aRegDate IN Registration.RegDate%TYPE,
aRegTerm IN Registration.RegTerm%TYPE,
aRegYear IN Registration.RegYear%TYPE,
aResult OUT BOOLEAN ) IS
-- aResult is TRUE if successful, false otherwise.
BEGIN
aResult := TRUE;
INSERT INTO Registration
(RegNo, StdSSN, RegStatus, RegDate, RegTerm, RegYear)
VALUES
(aRegNo, aStdSSN, aRegStatus, aRegDate, aRegTerm,
aRegYear);
EXCEPTION
WHEN OTHERS THEN aResult := FALSE;
END;
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Common Predefined Exceptions
Cursor_Already_Open
 Dup_Val_On_Index
 Invalid_Cursor
 No_Data_Found
 Rowtype_Mismatch
 Timeout_On_Resource
 Too_Many_Rows

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Format of PL/SQL Functions
CREATE [OR REPLACE] FUNCTION FunctionName
[ (Parameter1, …, ParameterN) ]
RETURN DataType
IS
[ sequence of declarations ]
BEGIN
sequence of statements including a RETURN statement
[ EXCEPTION
sequence of statements to respond to exceptions ]
END;
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Simple Function Example
CREATE OR REPLACE FUNCTION fn_RetrieveStdName
(aStdSSN IN Student.StdSSN%type) RETURN VARCHAR2 IS
aFirstName Student.StdFirstName%type;
aLastName Student.StdLastName%type;
BEGIN
SELECT StdFirstName, StdLastName
INTO aFirstName, aLastName
FROM Student
WHERE StdSSN = aStdSSN;
RETURN(aLastName || ', ' || aFirstName);
EXCEPTION
WHEN No_Data_Found THEN
RETURN(NULL);
WHEN OTHERS THEN
raise_application_error(-20001, 'Database error');
END;
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PL/SQL Cursors
Supports usage of SQL statements that
return a collection of rows
 Declaration statements
 Specialized FOR statement
 Cursor attributes
 Actions on cursors

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Classification of Cursors

Statement binding:
– Static: SQL statement specified at compile-time
– Dynamic: SQL statement specified at execution

Declaration status
– Implicit: declared, opened, and iterated inside a
FOR statement
– Explicit: declared with the CURSOR statement
in the DECLARE section
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Common Cursor Attributes
%ISOpen: true if cursor is open
 %Found: true if cursor is not empty
following a FETCH statement
 %NotFound: true if cursor is empty
following a FETCH statement
 %RowCount: number of rows fetched

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PL/SQL Packages
Larger unit of modularity
 Improved reusability
 Groups procedures, functions, exceptions,
variables, constants, types, and cursors.
 Public interface
 Private body: implementation of package
 Oracle provides predefined packages

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Format of Package Interface
CREATE [OR REPLACE] PACKAGE PackageName IS
[ Constant, variable, and type declarations ]
[ Cursor declarations ]
[ Exception declarations ]
[ Procedure definitions ]
[ Function definitions ]
END PackageName;
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Format of Package Body
CREATE [OR REPLACE] PACKAGE BODY PackageName IS
[ Variable and type declarations ]
[ Cursor declarations ]
[ Exception declarations ]
[ Procedure implementations ]
[ Function implementations ]
[ BEGIN sequence of statements ]
[ EXCEPTION exception handling statements ]
END PackageName;
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Trigger Overview
Event-Condition-Action (ECA) rules
 Managed by DBMS
 Execution controlled by inference engine
 DBMS extended with inference engine
 Part of SQL:1999
 Widely implemented before SQL:1999

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Typical Usage of Triggers
Complex integrity constraints
 Transition constraints
 Update propagation
 Exception reporting
 Audit trail

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Classification of Triggers

Granularity
– Row: fire for each modified row
– Statement: fire once per statement
Timing: before or after
 Event

– Manipulation statements
– Update event with a list of columns
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Format of Oracle Triggers
CREATE [OR REPLACE] TRIGGER TriggerName
TriggerTiming TriggerEvent
[ Referencing clause ]
[ FOR EACH ROW ]
[ WHEN ( Condition ) ]
[ DECLARE sequence of declarative statements ]
BEGIN sequence of statements
[ EXCEPTION exception handling statements ]
END;
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AFTER ROW Trigger Example
CREATE OR REPLACE TRIGGER tr_Enrollment_IA
-- This trigger updates the number of enrolled
-- students the related offering row.
AFTER INSERT
ON Enrollment
FOR EACH ROW
BEGIN
UPDATE Offering
SET OffNumEnrolled = OffNumEnrolled + 1
WHERE OfferNo = :NEW.OfferNo;
EXCEPTION
WHEN OTHERS THEN
RAISE_Application_Error(-20001, 'Database error');
END;
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Guide to Trigger Examples

BEFORE ROW:
– Complex integrity constraints
– Transition constraints
– Standardization of data

AFTER ROW
– Update propagation
– Audit trail
– Exception reporting
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Compound Events in Triggers

Compound events
– Use OR to specify multiple events
– Trigger body can detect the event

Multiple triggers versus compound event triggers
– More triggers but less complex
– Fewer, more complex triggers
– Trigger interaction increases with the number of
triggers
– No clear preference
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Trigger Execution Procedure
Inference engine that controls trigger firing
 Specifies execution order among triggers,
integrity constraints, and manipulation
statements
 Trigger body execution can cause other
triggers to fire
 SQL: standard trigger execution procedure
 Most DBMSs deviate from the standard

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Simplified Oracle Trigger
Execution Procedure
1. Execute the applicable BEFORE STATEMENT triggers.
2. For each row affected by the SQL manipulation statement:
2.1 Execute the applicable BEFORE ROW triggers.
2.2 Perform the data manipulation operation on the row.
2.3 Perform integrity constraint checking.
2.4 Execute the applicable AFTER ROW triggers.
3. Perform deferred integrity constraint checking.
4. Execute the applicable AFTER statement triggers.
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Overlapping Triggers

Definition:
– Two or more triggers with the same timing,
granularity, and applicable event
– Same SQL statement causes both triggers to
fire
SQL:1999: firing order based on trigger
creation time
 Oracle: arbitrary firing order
 Carefully analyze overlapping triggers

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Recursive Trigger Execution
1. Execute the applicable BEFORE STATEMENT triggers.
2. For each row affected by the SQL manipulation statement
2.1. Execute the applicable BEFORE ROW triggers.
Recursively execute the procedure for data
manipulation statements in a trigger.
2.2. Perform the data manipulation operation on the row.
2.3. Perform integrity constraint checking. Recursively
execute the procedure for actions on referenced
rows.
2.4. Execute the applicable AFTER ROW triggers.
Recursively execute the procedure for data
manipulation statements in a trigger.
3. Perform deferred integrity constraint checking.
4. Execute the applicable AFTER statement triggers.
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Controlling Trigger Complexity

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Do not use data manipulation statements in
BEFORE triggers.
Limit data manipulation statements in AFTER
triggers.
For triggers that fire on UPDATE statements,
always list the columns in which the trigger
applies.
Ensure that overlapping triggers do not depend
on a specific order to fire.
Be cautious about triggers on tables affected by
actions on referenced rows.
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Mutating Table Errors
Restriction on trigger execution in Oracle
 Mutating tables of a trigger:

– Table in which trigger is defined
– Related tables affected by CASCADE
DELETE
Oracle prohibits SQL statements in a
trigger body on mutating tables
 Run-time error during trigger execution

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Resolving Mutating Table Errors
Avoid by using new and old values
 Sometimes unavoidable

– Trigger to enforce integrity among rows of the
same table
– Trigger to insert a related row in a child table
with DELETE CASCADE

Resolutions
– Package and a collection of triggers
– Use INSTEAD OF trigger for a view
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Summary
Stored procedures and triggers are
important for database application
development and database administration
 Benefits for DBMS management of stored
procedures
 Classification of triggers by granularity,
timing, event, and purpose
 Knowledge of trigger execution
procedures

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